Ion source

ABSTRACT

An ion source ( 10 ) for producing a beam of ions from a plasma is disclosed. A plasma is created at the center of an annular anode ( 12 ) by collisions between energetic electrons and molecules of an ionisable gas. The electrons are sourced from a cathode filament ( 11 ) and are accelerated to the anode ( 12 ) by an applied electric potential. A magnetic field having an axis aligned with the axis of the anode acts to concentrate the flow of electrons to the center of the anode ( 12 ). The ionisable gas is introduced into the ion source ( 10 ) at the point of concentrated electron flow. Ions created in the resultant plasma are expelled from the ion source as an ion beam centered on the axis of the magnetic field. The surfaces of the anode are coated with an electrically conductive non-oxidising layer of Titanium Nitride to prevent a build up of an insulating layer on the anode.

BACKGROUND OF THE INVENTION

This invention relates to ion sources for producing an ion beam. Theinvention was developed through use with end-Hall effect ion sources andis, at times, described with particular reference thereto. It will beapparent to the skilled reader however, that the scope of the inventionwill encompass other types of ion sources.

Ion sources had their origins in space propulsion but more recently havefound use in more industrial processes such as Ion Assisted Deposition(IAD) of thin film coatings. In an IAD process, an ion beam from an ionsource is focussed onto a target substrate to cause densification of thecoating material as it is deposited. The process occurs within anevacuated chamber of pressure of the order 10 ⁻²Pa.

In a typical ion source, electrons are drawn from a cathode filamenttoward an anode through an ionisable gas. Collisions between the gasmolecules and energetic electrons create a source of positive ions byinducing a plasma. In one type of ion source known as a gridless ionsource, a magnetic field is applied across the plasma to shape the ionsaccelerated from the ion source into an ion beam. In a specific type ofgridless ion source, known as an end-Hall effect ion source, the axis ofthe magnetic field is aligned with the electric potential between thecathode and the anode. The interaction of the magnetic and electricfields causes the charged particles to approximately follow the magneticfield lines. The anode in these devices is typically annular having anoutwardly inclined inner diameter with the bulk of the plasma formingwithin the confines of the anode walls.

An example of the an end-Hall effect ion source in common use, inparticular in IAD techniques, is described in U.S. Pat. No. 4,862,032 toKaufman et al. In this device, herein referred to as the Kaufman device,the ionisable gas is distributed uniformly across the plasma region.Magnetic field shaping disperses the electrons across the gas to ensurea large plasma capable of producing a high ion beam current. The resultis that a relatively high gas flow (typically up to 50 sccm) is requiredto maintain a sufficient pressure in the plasma region to achieveionisation of the gas. The resultant high background pressure within theinterelectrode space creates electrical instability leading to thegeneration of cathode spots within the ion source and extending to theextremities of the vacuum environment. In addition, large vacuum pumpsare required to maintain a sufficiently low pressure within the rest ofthe evacuated chamber to be compatible with the operation of otherequipment used in IAD and other processes. In operation the pressure canonly be increased to the point where the ion beam current isapproximately 1 Amp before further instabilities are introduced.

A further problem with present ion sources is that their performance candecrease over the life of the ion source. Symptoms include difficulty inestablishing the plasma and a reduced stability of the plasma.Investigations by the present inventor have found that the reducedperformance capabilities are created, at least in part, by a decrease inthe electron flux entering the ionisation region due to a reduction inthe effective surface potential of the anode. Further investigation intothe cause of the reduced potential by the present inventor found that adielectric oxide layer built up on the surface of the anode exposed tothe plasma. It was previously believed that the observed build up ofelectrically insulating coatings on the anode were produced byscattering and sputtering from the thin film deposition processes forwhich these ion sources were commonly used. The inventor has found thatthe dielectric layer actually arises from a small percentage of negativeions produced in an oxygen plasma interacting with the surface of theanode and that this has the effect of shielding the anode from thecathode, dispersing the electron flow from the cathode and thus reducingthe electron flux into the ionisation region. The reduced electron fluxinto the ionisation region firstly creates instability in theperformance of the ion source and, secondly, causes an imbalance in thechange neutrality of the resultant ion beam.

SUMMARY OF THE INVENTION

In a first form, the present invention resides in an ion sourceincluding a cathode, an anode, an ionisation region between said cathodeand said anode, means for introducing an ionisable gas into saidionisation region, means for creating a potential difference betweensaid cathode and said anode to produce a flow of electrons from saidcathode toward said anode, said electron flow passing substantiallythrough said ionisation region and causing ionisation of said gas, meansfor concentrating said electron flow to create a region within saidionisation region where the electron flux is a maximum, and means actingto expel ions created in said ionisation region from said ion source,wherein said ionisable gas is introduced into said ionisation region ata localised area in proximity to said region of maximum electron flux.

Preferably the concentration of electrons and the expulsion of ions fromthe ion source is achieved using a magnetic field.

More preferably, the axis of the magnetic field lies substantiallyparallel to the direction of the electric potential between the anodeand the cathode. With the magnetic and electric fields aligned in thisway, the maximum electron flux occurs at the maximum magnetic fieldintensity.

The invention also provides an ion source including a cathode, an anode,an ionisation region between said cathode and said anode, means forintroducing an ionisable gas into said ionisation region, means forcreating a potential difference between said cathode and said anode toproduce a flow of electrons from said cathode toward said anode, saidelectron flow passing substantially through said ionisation region andcausing ionisation of said gas, and means acting to expel ions createdin said ionisation region from said ion source, wherein said anode hasat least one surface exposed to said ionisation region, at least aportion of said at least one exposed surface being of an electricallyconducting non-oxidising material.

Preferably the anode is annular having an axis lying in the samedirection as the electric field between the anode and the cathode. Theexposed surfaces of the anode are preferably a coating of TitaniumNitride (TiN).

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will become apparent tothe skilled reader from the following description of preferredembodiments made with reference to the accompanying Figures in which:

FIG. 1 is a partial cross-sectional elevation of the ion sourceaccording to the invention.

FIG. 2 is a plan view of the ion source in FIG. 1.

FIG. 3 is a cross-sectional view of a preferred form of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIGS. 1 and 2 show an ion source generally at 10 having a cathode wire11 and an anode 12. The anode 12 is an annulus having an inner surface35 sloping outwards in the direction of the cathode. Between the cathode11 and the anode 12 is an ionisation region 13. The cathode wire 11 issuspended above the anode by two mounting pins 20 that are held by, andin electric isolation from a shield plate 30. The shield plate 30substantially surrounds the anode, cathode and ionisation region byextending from a point lower than the anode 12 to a point above thecathode 11 and is preferably maintained at earth potential to shield theanode and the cathode from external fields. A magnet 14 is disposedoutside the ionisation region 13 but adjacent the anode 12. The magnet14 creates a magnetic field, longitudinal axis of which is aligned withthe axis of the anode 12. The magnet may be a permanent magnet or anelectromagnet. Preferably the magnet is a high flux rare earth magnetsuch as a NdFeB magnet. As an alternative, magnet 14 may be a ringmagnet disposed around the anode 12 and ionisation region 13.

The alignment of the magnetic field with the electric field causeselectrons emitted by the cathode to approximately follow the magneticfield lines as they move towards the anode. This has the effect ofconcentrating the flow of electrons toward the axis of the magneticfield. Therefore the region where the magnetic field intensity is amaximum, will also be a region of maximum electron flux.

The ionisable gas, for example oxygen, nitrogen or argon, is supplied tothe ionisation region through a gas flow path from gas feed line 22. Thegas flow path terminates at an outlet member 15. The outlet member 15has the form of a gas shower head, with a plurality of apertures 17,that introduce the gas into the ionisation region 13 in a substantiallyrandom direction. The gas shower head 15 is disposed on the axis of theanode and adjacent the ionisation region 13 such that gas emanating fromthe apertures 17 enters the ionisation region at a point of highelectron flux. Because a large proportion of ionisation occurs close tothe outlet, the gas shower head is of a material such as stainlesssteel, that withstands the very high energy from the incoming electronflux.

The anode 12 preferably has disposed within it a channel 53 incommunication with a fluid conduit 55 that provides water to cool theanode. The channel 53 preferably extends into the body of the outletmember 15.

The anode 12, outlet member 15 and shield 30 are mounted on a nonconductive mounting base 50 through which extends the gas flow path andfluid conduit 55. A plurality of mounting screws 57 fix the anode 12 tothe base 50. The magnet 14 is housed within the base such that theexternal pole is exposed. The mounting base 50 has a conduit 58 thatforms part of the gas flow path and connects the gas feed line 22 to theoutlet member 15 such that no electrical connection can be made betweenthe outlet member 15 and the gas feed line 22. The mounting base 50 hasa similar conduit for connecting the water feed line 55 to the channel53. The gas and water feed lines preferably screw into the mounting base50. A suitable material for the mounting base 50 is glass filledpolytetrafluoroethylene. This arrangement reduces electrical hazards,simplifies mounting and installation and reduces risk of secondaryplasmas forming within the gas feed line.

The size of the outlet is preferably half or less than the smallestinner diameter of the anode in order that a localised high pressure zoneis created around the outlet, that decreases rapidly with distance.

In operation the anode is charged in the range 0-500 V preferably 250 Vrelative to the cathode which is at or near earth potential. A DCcurrent of approximately 12A is passed through the cathode to stimulateelectron emission. An AC current may be used but the combination of analternating current and the magnetic field has been found to causevibrations in the cathode which reduces the cathode lifetime. Electronsgenerated at the cathode are influenced by the anode potential and areaccelerated toward it. The magnetic field imparts a spiral motion on theelectrons further increasing their potential to ionise gas molecules andfocussing the electrons toward the longitudinal axis. Collisions betweenthe energetic electrons with gas molecules emitted from the outletmember 15 cause ionisation. If sufficient ionising collisions occur thena plasma is formed. Positive ions created in the plasma experience theopposite effect to the electrons. The ions initially have a randomvelocity but are influenced by the potential gradient which acceleratesthem toward and past the cathode 11. The magnetic field in this caseacts to control the direction in which the ions are expelled from theion source by focusing them into an ion beam centred on the longitudinalaxis of the magnetic field. The dynamics of the interactions between theions and the electric and magnetic fields for this configuration areknown per se, for example from the above mentioned Kaufman patent. Thecurrent of the ion beam is effected by the size of the plasma which canbe controlled by the gas flow rate.

The plasma can be maintained for a wider range of gas flow rates thanfor prior art ion sources because there is always at least a localisedregion of high pressure. The range of gas flows gives a correspondingrange in the ion beam currents. A further advantage is that lower gasflow rates are required to achieve the equivalent or higher beamcurrents than for prior art devices. For example a gas flow rate of 4-5sccm can achieve a beam current of 2 A in the present invention comparedwith 10-50 sccm required to produce 1 A of beam current in devices ofthe above mentioned Kaufman type. These lower gas flow rates assist inallowing a low background pressure to be maintained.

A further benefit of reduced flow rate is that the operationalrequirements of the vacuum pumping system used to evacuate the chamberin which the ion source is disposed can be reduced, while stillmaintaining lower background pressures than achieved in many prior artdevices. This increases stability by reducing the chances of arcing andsputtering in the peripheral regions of the ion source.

Operating background pressures of the order 10⁻³ Pa have been achievedwith the present invention. At these pressures the mean free path of theions is of the order of metres. This is important in many ion sourceapplications because it is typically many times longer then thedimensions of the vacuum environment. For IAD processes, mean free pathsof this order are longer than the typical distance between the ionsource and the target substrates. The efficiency of the depositionprocess is therefore enhanced by these low background pressures becausemore primary ions impact the target substrates instead of undergoingsecondary collisions with gas molecules. A further benefit of thereduced pressure is that contamination of the thin film coating, isconsiderably reduced.

The anode 12 is preferably made of stainless steel but has a coating ofa non-oxidising electrically conductive material, for example TiN, onthe inner surface 35 and any other surface that in use may be exposed tobombardment by electrons and/or negative ions from the plasma. The innersurface coating is unreactive with any negative ions produced in theplasma and therefore resists the build up of a dielectric layer on theanode surface. This provides a long term benefit in the performance ofthe ion source because a dielectric coating would shield the anodepotential from the cathode. This would reduce the concentration ofelectrons flowing into the ionisation region, thus reducing the size ofthe plasma and in turn the ion beam current. In addition, theconcentration of electrons in peripheral regions of the ion source wouldincrease, thereby increasing the frequency of arcing and sputtering inthese regions. By coating the anode in a non-oxidising material, theseproblems can be eliminated as can the cleaning procedures previouslyrequired to maintain the anode in working order.

Because the ion source 10 operates at a low background pressure theanode and cathode can be in closer proximity than in previous devices.FIG. 3 shows a preferred form of the invention where the inner edge 31of the plasma shield 30 extends towards the anode 12. Preferably theinner edge 31 of the shield 30 is disposed outside a projection of theinner surface 35 of the anode 12. The extended edge 31 has a flange 32that surrounds an upper portion of the anode 12. The purpose of theflange 32 is to prevent gas entering the region 40 enclosed by the anode12 and shield 30 where the gas could be ionised and cause electricalinstability. A vent hole 41 is provided from the region 40 to outsidethe ion source to allow sufficient pumping of this region, thus ensuringa low pressure To further prevent any instabilities an o-ring seal (notshown), preferably of an elastomer material, can be disposed between theflange 32 and an upper portion of the anode 12.

While particular embodiments of this invention have been described, itwill be evident to those skilled in the art that the present inventionmay be embodied in other specific forms without departing from theessential characteristics thereof. The present embodiments and examplesare therefore to be considered in all respects as illustrative and notrestrictive, the scope of the invention being indicated by the appendedclaims rather than the foregoing description, and all changes which comewithin the meaning and range of equivalency of the claims are thereforeintended to be embraced therein.

What is claimed is:
 1. An ion source including a cathode, and anode, and ionisation region between said cathode and said anode, means for introducing an ionisable gas into said ionisation region, means for creating a potential difference between said cathode and said anode to produce a flow of electrons form said cathode toward said anode, said electron flow passing substantially through said ionisation region and causing ionisation of said gas, and means acting to expel ions created in said ionisation region from said ion source, wherein said anode has at least one surface exposed to said ionisation region, at least a portion of said at least one exposed surface comprising a layer of Titanium Nitride.
 2. An ion source according to claim 1 wherein said anode is annular and includes an inner surface sloping outwards in the direction of said cathode, said inner surface being exposed to said ionisation region and at least a portion of said inner surface comprising a layer of Titanium Nitride.
 3. An ion source according to claim 2 wherein substantially the entire inner surface of said anode is comprised of a layer of Titanium Nitride.
 4. An ion source according to claim 2 wherein said gas introducing means includes an outlet member disposed substantially at the centre of said anode, said outlet member having a surface comprising a layer of Titanium Nitride. 